The present invention relates to a control method by which an optical output level to be set by the number of channels of wavelength multiplexing signal light can be changed to suppress a variation in optical output level. More particularly, the present invention relates to a channel counter that checks for the number of wavelength multiplexing light channels to perform the above-mentioned control method.
Wavelength multiplexing transmission is the scheme of using signal light containing plural wavelengths in optical communications to expand the transmission capacity. This scheme requires optical multiplexing in the transmission section optical demultiplexing in the receiving section, so that loss occurs due to optical multiplexing and demultiplexing. In many cases, optical amplifiers are used to compensate the optical multiplexing and demultiplexing loss or the transmission path loss.
A semiconductor amplifier, a fiber Raman amplifier, a fiber Brillouin amplifier, or a rare-earth-element-doped-fiber amplifier is listed as an optical amplifier. Particularly, the erbium-doped-fiber amplifier (EDFA), or a type of rare-earth-element-doped-fiber amplifier, is often used because it has no polarization dependent characteristic and can be pumped by a semiconductor laser. The EDFA amplifier generally controls its optical output to a fixed level.
FIG. 3 is a block circuit diagram illustrating the configuration of an ordinary EDF amplifier. In this example, the optical branching coupler 13 inserted in the output portion of the EDF amplifier 12 partially branches wavelength multiplexing light. The photo diode (PD) module 15 converts the branched light into a current. The optical amplifier control circuit 16 controls the pumping laser module 14 to set the current to a constant value. Thus the optical output is controlled to a constant level.
However, a fixed number of signal light beams are not always transmitted in the wavelength multiplexing transmission. For example, provided that the optical output level to be set is 40 mW, when wavelength multiplexing signal light beams are input on four channels, the optical output per channel is 10 mW. When the wavelength multiplexing light beams on two channels are input, the optical output per channel is 20 mW. That is, the optical output level per channel depends on the number of channels of wavelength multiplexing signal light beams to be input.
In order to suppress changes in the optical output level, there is the control method in which the optical output level is varied to a set value according to the number of channels for wavelength multiplexing light. This control method requires a channel counter that checks for the number of channels for wavelength multiplexing light.
FIG. 4 is a block circuit diagram illustrating the configuration of an ordinary channel counter. In this example, an 1.times.4 optical branching coupler 17 divides wavelength multiplexing signal light input to the channel counter into four. Each optical filter module 18 extracts the signal light of each wavelength. Each photo diode (PD) module 15 converts each signal light into a current. The channel counter 19 counts the current.
However, since the above-mentioned prior art requires the optical branches, fiber gratings, and optical receivers, each corresponding to the channel number to be counted, the configuration of the optical circuit becomes complicated.
The prior art counter cannot count more than the number of channels corresponding to optical branches, thus it is not capable of dealing with an increasing number of channels.
Moreover, since the prior art counter can count only the signal light of the transparent wavelength of the optical filter, it cannot deal with a variation in layout of the wavelength on each channel.